Gas Turbine engines may utilize a dual annular combustion system which consists of an outer (pilot) dome and an inner (main) dome. In a dual annular combustor, fuel is supplied to fuel nozzles in both the pilot dome and the main dome. In controlling fuel flow to the main and a pilot domes, it is essential to ensure that flow is properly allocated over a wide range of total fuel flows. Control of the distribution of engine fuel flow between the two domes enables the combustor performance to be optimized for particular engine operating conditions. For example, the fuel split between the domes can be selected to minimize the emissions of the engine and protect the engine operability limits (i.e. flameout and temperature limits). The distribution of fuel between the main and pilot domes may be controlled as a function of typical engine parameters (e.g., total fuel flow, inlet temperature or pressure).
One such scheme for providing the variable flow split of a fully modulated burner staging valve is described in U.S. Pat. No. 4,691,730. The flow divider, presented in U.S. Pat. No. 4,691,730 uses a flow control (or divider) valve and a pressurizing/backpressure equalizing shuttle valve. The flow control valve is operated as a function of the desired flow split. Reliable and accurate flow splits can be expected for a 4:1 inlet flow range. Typically, in gas turbine engines, minimum to maximum total fuel flow will have a range of approximately 60:1. It would, therefore, be advantageous to design a burner staging valve capable of accurate flow splits for variations in total flow in the range of approximately 60:1.
Flow split is also important because a flow split biased to give too much flow to either zone of the combustor will result in a skewed temperature profile in the high pressure turbine at the combustor outlet. A skewed temperature distribution is undesirable. At lower power levels an incorrectly biased fuel flow split could allow the fuel to air ratio in one of the domes to be too low to maintain combustion. At high power levels an incorrectly biased fuel flow split could result in localized overheating in the turbine. Therefore, it would be advantageous to design a failsafe burner staging valve to prevent these problems.